There are virtually endless applications for wear-resistant coatings. For example, mechanical parts that are subject to moving contact with other parts will eventually wear to the point that the part must be reshaped or replaced. Similarly, cutting edges, when dulled, must be resharpened or replaced. Each of these applications would benefit greatly by the use of a wear-resistant coating that is thin enough not to substantially alter the part size, has good adhesion to the substrate so it does not flake or chip in use, and is extremely smooth so that it does not increase the friction of the coated part.
There are additional applications in which wear-resistant coatings would be extremely useful in protecting an underlying part to increase the part life by acting as a barrier layer and a wear-resistant layer. For example, manufactures of plastic eyeglass lenses use lens molds that must be extremely smooth, wear-resistant, and able to easily release the molded lens. Other substrates such as long-life digital recording media compact discs require a coating that is optically transparent, wear-resistant, and will act as a moisture barrier to protect the underlying material containing the encoded information from degradation.
Many processes have been developed to provide wear-resistant coatings for these varied applications. One commonly employed process coats parts with titanium nitride using an ion plating process that employs an intense arc to evaporate the titanium target as titanium ions. The surface to be coated is held near the target at ground so that the titanium ions impact the surface at high energy. Typically, this process takes place in a nitrogen atmosphere for supplying ionized nitrogen that is similarly accelerated into the surface, where it reacts with the titanium ions to form the titanium nitride coating. These coatings, however, are typically smooth to only approximately a few thousand Angstroms and thus are insufficient for applications requiring extremely smooth coatings. In addition, the substrate temperatures of several hundred degrees celsius make these high temperature processes inappropriate for materials with low melting temperatures such as plastics, substrate materials that cannot stand the thermal stress associated with high temperature operation, or substrate-coating combinations that are mismatched in coefficient thermal expansion so that unacceptable stresses are introduced when the part is cooled from the high deposition temperature.
Another coating deposition process employs RF plasma chemical vapor deposition in which two closely spaced electrodes are used to create an electric field between the electrodes in which a plasma is formed. The substrate is placed in or adjacent to the plasma for chemical vapor deposition of coating compounds. This CVD process, however, operates at a high voltage, which would cause sputtering of the substrate surface, resulting in an unacceptably rough surface in any applications requiring an extremely smooth surface. Further, this RF plasma process is not suitable for coating pointed or irregularly-shaped objects, as non-planar surfaces may cause arcing between an electrode and the surface.
Silicon carbide is an extremely hard compound that is a commonly used abrasive which exhibits hardness and wear characteristics superior to many other abrasives. Silicon carbide films have been found to be useful x-ray lithography windows due to the mechanical properties and transparency of the compound. These silicon carbide x-ray lithography windows are typically fabricated by depositing the silicon carbide on a silicon substrate using microwave-energized electron cyclotron resonance (ECR) apparatus. This process typically takes place at a relatively high temperature of approximately 500.degree. C. to 800.degree. C. to create films of low tensile stress so that the window created by etching away part of the silicon substrate from the backside after deposition of the silicon carbide layer is under sufficient tension that it will not buckle.